Various raw materials containing zinc are discharged in the iron and steel industry and nonferrous metal industry. However, most of them are discarded because of their low contents of zinc and higher contents of iron in comparison of the zinc contents and consequently many problems in public circumstance protection have been brought out.
In the iron and steel industry, these materials are produced mainly in dust collectors treating exhaust gas from high temperature portions of blast furnaces, converters and electric furnaces, etc. While, in process industries, these materials are produced mainly in portions when scums formed in upper and under parts of fused zinc galvanizing tanks and fine dusts generated from the total surface thereof are treated and further dust collectors which exhaust gas from scrap-treating shops typically presented by junked car scrapping shops. In the nonferrous metal industry, these materials are produced from processing of leached residues in hydrometallur-gical refining processes, sludges in solution purifying processes, slags and exhaust gas in pyrometallurgical processes. However, as described above, valuable metals such as zinc, etc. cannot be recovered from these materials due to high contents of iron and these materials have long been wasted and discarded.
When the iron and zinc ions are contained in an aqueous solution, it has been known for separation of their ions that iron hydroxide is produced and removed by controlling pH values but this method has a disadvantage that zinc recovery is decreased due to a large amount of zinc which coprecipitates with iron hydroxide. Although the raw materials containing iron and zinc as mentioned above can be dissolved by acids, economical zinc recovery is very difficult due to coprecipi-tation of zinc with iron and consequently these materials have been discarded.
Norwegian Institute of Technology proposed a zinc recovery process for effective separation of zinc and iron from raw materials containing them without using acid. According to this process, ZnO in the raw materials is selectively leached in contact with an organic solvent containing Versatic acid without leaching. EQU ZnO.sub.(Solid) +2RH.sub.(Org) .revreaction.R.sub.2 Zn.sub.(Org) +H.sub.2 O.sub.(Aq)
Furthermore, a process for recovering organic solvents remaining in large amounts in leached residues has been proposed in which the residues are washed with an aqueous solution containing NaOH and Na.sub.2 CO.sub.3 and then with hot water. In this process, however, the residues containing the organic solvent become massive and consequently it is very difficult to recover the adhering organic solvents from the residues.
Typical samples of zinc containing raw materials discharged from the iron and steel industry are shown below.
______________________________________ Fe Zn Pb Ca Na Al.sub.2 O.sub.3 SiO.sub.2 C ______________________________________ Electric furnace 31.8 18.4 3.1 2.5 1.9 0.83 4.8 11.5 dust: Blast furnace 35.2 1.6 0.3 3.7 -- 2.3 5.4 33.4 dust: ______________________________________ (in %) ______________________________________
As understood from the above, leaching process with an organic solvent containing versatic acid mentioned above is not economical for treating the materials, because of their low contents of zinc, large amounts of ZnO.Fe.sub.2 O.sub.3, particularly in electric furnace dusts, and an increased loss of organic solvent adhered in the leached residues. The present inventors proposed a process for recovery of organic solvents adhering in residues, in which the residue is washed with isopropyl alcohol and acetone and then the organic solvent is recovered by fractional distillation of them. However, this process could not be industrially used owing to its complexity and high recovery cost.
Also a process is known in which iron making raw materials and raw materials containing large amounts of zinc are recovered from raw materials of low zinc content, such as blast furnace dust, by heating at 400.degree.-1600.degree. C. in a reducing atmosphere and volatilizing zinc and has been commercially adopted but the economical advantage of this process has been decreased by extraordinary enhancement in price of heating fuels.
For these and other reasons, raw materials containing less than 50% of zinc are not accepted by nonferrous smelters due to the economical disadvantages and are at present discarded.
This invention proposes a resource cycle system for treating economically liquid or solid raw materials which contain relatively higher contents of iron incomparison with zinc contents and cannot be economically treated by the conventional methods.